Magnetocaloric Effects And First-principle Calculations Of Ce₂Fe₁₇and La（Fe,Si）₁₃ Series Compounds

Rare earth-iron based compounds have great potential for applications in magnetic refrigeration and ice melting because of their adjustable phase transition temperature,obvious magnetocaloric effect and low raw material cost.At present,the high-abundance rare earth iron based compounds(Ce₂Fe₁₇,La（Fe,Si）₁₃)suffer from the problems of low phase transition temperature,magnetocaloric effect still needs to be optimized and large thermal hysteresis of the first order phase transition.This thesis combined experimental studies and theoretical calculations to develop rare earth iron based compounds with good magnetocaloric properties,deeply investigated the inner connection between composition,cell structure,electronic structure and magnetocaloric effect,revealed the regulation mechanism of phase transition temperature and phase transition order,which provided a theoretical basis for the composition design and performance optimization of rare earth iron based compounds.The main research findings are as follows:（1）Ce_1.6Nd_0.4Fe_17-xSi_x（x=0,0.33）alloys with excellent room temperature magnetocaloric properties were prepared.Its phase transition temperatures(T_trans)were 273.0 K and 297.4 K,the maximum magnetic entropy changes(|ΔS_M|_max)were 4.1 and 3.7 J?kg^-1K^-1,and relative cooling powers（RCP）were 371 and 350 J?kg^-1（0～5 T）,respectively.In combination with the first-principle calculations,it was found that both Nd and Si lead to an increase in the Fe atomic spacing between the 6c sites,raising the phase transition temperature.Si was found to lower the formation energy and promote the formation of the2:17R phase through the hybridization effect and covalent-like bonding properties between Si 3p and Fe 3s orbitals.（2）The addition of the magnetic element Co was found to simultaneously increase the phase transition temperature and the magnetocaloric effect of Ce₂Fe_17-xCo_x alloys,achieving a controlled adjustment of the phase transition temperature in the room temperature range.As the Co content increases from 0.33 to 1,the T_trans increases linearly from 261.9 K to 335.6 K,the|ΔS_M|_max increases from 2.3 to2.6 J?kg^-1K^-1（0～2 T）,and the RCP is increased from 164.8 to 171.3J?kg^-1.It is found that Co increases the phase transition temperature via increasing the exchange interaction parameter,and increases the magnetocaloric effect via enhancing the phase transition rate and introducing a"spike"at the Fermi energy level in the density of states.（3）The phase transition order and magnetocaloric effects of the La_1-xCe_xFe_11.2Si_1.8（x=0,0.1,0.2,0.3）alloys are adjusted by modifying the La/Ce ratio.The Ce content of 0.16 was determined as the critical composition for the first order phase transitions（FOPT）and second order phase transitions（SOPT）.Its T_trans is 194.4 K,thermal hysteresis(T_hys)is 1.2 K,and the|ΔS_M|_max is 12.2 J?kg^-1K^-1（0～2 T）.It is found that Ce decreases the phase transition temperature by reducing the gap between icosahedral atomic clusters,but maintains the magnetic-volume coupling during the phase transition.The cell volume contraction caused by Ce shifts the spin-down minimum in the total DOS near the Fermi energy level,enhancing the Itinerant electron metamagnetic（IEM）transition feature and transforming the SOPT into FOPT.（4）The effects of interstitial C on the microstructure,phase structure,magnetic phase transition and magnetocaloric effects of La Fe_11.5Si_1.5C_x alloys are investigated.The C leads to an increase in the T_trans and transform the FOPT into SOPT.The C content of 0.14 is determined as the critical composition for the FOPT and SOPT,with T_trans of 212.5 K,T_hys of 2.5 K,and|ΔS_M|_max of 13.2 J?kg^-1K^-1（0～2 T）.It is found that the chemical pressure of interstitial C increases the gap between icosahedral atomic clusters leading to an increase in the phase transition temperature.Moreover,the hybridization of C 2p with Fe 3d orbitals and the nature of the polar covalent bonding-like interaction between Fe-C weaken the characteristics of the IEM transition,leading to a gradual change from FOPT to SOPT.（5）The application of Ce₂Fe₁₇ and La（Fe,Si）₁₃ based alloys in ice melting are explored.It is found that the surface of the test wire did not freeze under the condition of-10℃,and the temperature of the wire is maintained at about 5℃,which shows a good anti-icing and ice-melting effect.It indicates that the FOPT magnetocaloric materials with T_transnear 0℃have a large potential for ice melting applications.